Monitoring gas impurities with total sulfur detection

ABSTRACT

A system and method for determining impurities in a beverage grade gas such as CO2 or N2 relies on FTIR gas analysis for measuring non-sulfur impurities as well as SO2. CO2% also can be determined. A multiplexer selects a sample gas from multiple gas samples. Conversion of reduced sulphur present in some impurities to SO2 is conducted in an oxidizing furnace. Climate control and measurements of oxygen gas impurities also can be provided.

RELATED APPLICATIONS

This application claims the benefit under 35 USC 119(e) of U.S.Provisional Application No. 63/290,416, filed on Dec. 16, 2021, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Carbon dioxide (CO₂) is a colorless, odorless gas that can be used as aninert material, as pressurized gas, in “dry ice”, liquid orsupercritical fluid applications, and many other areas, such as, forinstance, oil production and the chemical industry. In the food sector,CO₂ is a medium for decaffeination and a feedstock for obtainingcarbonated beverages, providing effervescence to water, soft drinks,wine, beer and so forth. Applications in the beverage industry requireCO₂ of a specified purity. It is important, therefore, to monitor thenature and levels of contaminants in the gas employed.

Some existing systems for analyzing impurities in CO₂ gas rely on gaschromatography (GC) with photoionization detection (PID) and/or flameionization detection (FID). GC systems, however, can be slow, requiringseveral (e.g., 6-8) minutes between samples.

Other approaches rely on mass spectrometry (MS), a technique that isfast but can suffer from cross interferences and calibration issues.Continued maintenance often is required.

Specialized instrumentation geared toward detecting a particularcontaminant (total sulfur, for instance) or a class of contaminants(e.g., aromatics) also have been developed. These approaches, however,provide limited information. A sensor designed to focus on aromaticcompounds, for instance, may fail to signal the presence of acetaldehydeor nitrogen oxides (NO_(x)). In many cases, one or more additionaldevices are needed to analyze for other contaminants. Combining multipleinstruments often results in complicated designs, cumbersomecalibrations and extensive maintenance, increasing costs.

U.S. Pat. Nos. 10,408,746 B2 and 10,761,018 B2, both issued to Spartz etal. and incorporated herein in their entirety by this reference,describe approaches for determining impurities in a beverage grade gassuch as CO₂ or N₂ by coupling FTIR analysis and UV fluorescencedetection. Reduced sulfur present in some impurities is converted to SO₂in a furnace.

SUMMARY OF THE INVENTION

The food and beverage industry continues to demand techniques formeasuring a wide variety of impurities, often present at parts permillion (ppm) or even parts per billion (ppb) levels in gases such ascarbon dioxide or nitrogen. Streamlined and user-friendly systems arehighly desirable as are fast, in-line techniques. Some applications alsobenefit from capabilities for determining the quality of the gas used,the total % CO₂ in a beverage quality bulk CO₂ gas, for instance. Inmany cases, analyzing multiple (two or more) streams, sourced fromdifferent points in a manufacturing process is of great interest. Alsoof interest is conducting such analyses with simplified protocols andinstrumentation that can still provide comprehensive impurityinformation.

Generally, the invention relies on infrared absorption analysis such asFourier transform infrared (FTIR) gas analysis to measure impurities,including sulfur-based impurities, in beverage grade gases (such as CO₂and N₂, for example) or in other applications that place purityrequirements on a gas.

In many of its aspects, the invention features a system that includes aFTIR analyzer provided with a detector sensitive over a wideelectromagnetic energy range. In one embodiment, the detector (adeuterated triglycine sulfate (DTGS) detector, for instance) isresponsive over the entire mid-IR spectral range (wavelengths of 2 to 20microns).

The system also includes an oxidizer module for converting reducedsulfur present in a gas sample to SO₂. In specific implementations, theoxidizer includes a furnace that can be operated at a temperaturesufficient to carry out the oxidation reaction(s), e.g., about 1,000° C.

Also present is a multiplexer (or selector) module, typically a devicethat selects one sample input from among different input samples andforwards the selected sample for analysis. In specific examples, themultiplexer is a multi-position valve that steps incrementally throughcontinuous revolutions. Several channels (e.g., 4 to 10, or more) can beprovided for selecting a specific gas sample feed, which is thenanalyzed. In one embodiment, the sample feeds directed to themultiplexer are derived from multiple points in a plant employing abeverage grade gas.

An optional oxygen module for low level oxygen contaminations is addedin some cases. The system can further include a climate controlarrangement for maintaining a stable temperature, a parameterparticularly important when measuring very low impurity levels.

Thus, in one embodiment, a system for measuring impurities in a beveragegrade gas includes: a multiplexer for selecting a gas sample frommultiple gas samples; a spectrometer including a gas cell for detectingan absorbance spectrum of gas in the gas cell; an oxidizing furnace forconverting reduced sulfur present in the gas sample to SO₂; anarrangement including a device for directing the gas sample to thespectrometer or to the oxidizing furnace; and a computer module foroperating the multiplexer, the oxidizing furnace and/or the device fordirecting the gas to the spectrometer or to the oxidizing furnace.

In other aspects, the invention features a method for measuringimpurities in beverage grade gas such as CO₂, for example. In themethod, the impurities, including those that contain sulfur are detectedby a FTIR analyzer. In many embodiments, reduced sulfur species areoxidized to produce SO₂.

Accordingly, in another embodiment, a method for analyzing a beveragegrade gas comprises: (a) selecting a gas sample from multiple gassamples; (b) directing a first stream (or portion) of the gas sample toa gas cell; (c) measuring impurities present in the gas cell with aspectrometer, wherein the impurities include SO₂; (d) directing a secondstream (or portion) of the gas sample to an oxidizing furnace; (e)converting reduced sulfur present in the second stream (or portion) toSO₂; (f) directing gas exiting the oxidizing furnace to the gas cell;(g) measuring a total SO₂ in the gas cell; and (h) repeating steps (a)through (g).

It was initially thought that eliminating the UV fluorescence analyzerdescribed in U.S. Pat. Nos. 10,408,746 B2 and 10,761,018 B2 wouldpresent some disadvantages. Embodiments described herein address some ofthose concerns, resulting in a system and method that do not rely on UVfluorescence detection. Rather, all the normal contaminants (includingnon-sulfur as well as sulfur-containing compounds) specified by theInternational Society of Beverage Technologists (ISBT) are analyzed byFTIR spectrometry, using a single instrument, fitted with a singledetector.

In the beverage as well as other industries, the tolerance for manyimpurities present in gas streams can be very low, in the range of partsper million (ppm) or even parts per billion (ppb). The system and methoddescribed herein can offer a sensitive assessment of trace amounts whileproviding simultaneous readings for multiple contaminants. Both organic(e.g., volatile organic compounds or VOCs) as well as inorganicimpurities can be detected. Amounts of all the aromatics and aliphaticscan be correctly summed up. Methane can be measured individually.Approaches described herein can detect and measure moisture, a criticalcontaminant that can be introduced during truck delivery. Levels of SO₂,total sulfur and total reduced sulfur also can be determined. In oneexample, SO₂, total sulfur content as well as total reduced sulfur (TRS)can be measured down into the 10 s of ppb range.

Importantly, the quality of the CO₂ gas, a quality that can be affectedby the presence of nitrogen gas (N₂) or air, can be measured to reportCO₂ percentages in the gas being sampled. In contrast to other existingapproaches, practicing embodiments of the invention can provide CO₂measurements at 100%+/−0.03%.

The equipment described herein has at least one and preferably severalsample inputs, to handle, for example, truck delivery, bulk gas,purified bulk gas and/or other feeds. In some cases, input process gasstreams from different points of a carbonation process or plant (e.g.,delivery tanker, pre- or post-filtration and so forth), along with zerogas and validation gas are controlled automatically. If desired, thesystem and method described herein can be integrated into the plantdesign.

Practicing embodiments of the invention can offer fast measurementtimes, reduced calibration requirements, analysis of many compounds andthe capacity to measure multiple species using a single technique,namely FTIR spectrometry. The system can offer a fully integratedimpurity and CO₂% measurement system. In many cases, the response timecan be down to 5 seconds. Typically, calibrations for the FTIR data arenot needed. Implementations described herein do not require differentspectroscopic techniques or equipment or even switching betweendifferent FTIR detectors, offering a significantly streamlined and userfriendly approach.

Three to five times lower minimum detection limits (MDLs), an importantfeature for benzene which has the lowest MDL requirement of 20 ppbv(parts per billion by volume) can be reached. MDL or Method DetectionLimit is defined by the Environmental Protection Agency (EPA) as theminimum concentration of a substance that can be measured and reportedwith 99% confidence that the analyte concentration is greater than zero;it is determined from analysis of a sample in a given matrix containingthe analyte.

The above and other features of the invention including various detailsof construction and combinations of parts, and other advantages, willnow be more particularly described with reference to the accompanyingdrawings and pointed out in the claims. It will be understood that theparticular method and device embodying the invention are shown by way ofillustration and not as a limitation of the invention. The principlesand features of this invention may be employed in various and numerousembodiments without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a block diagram presenting the components (modules) of asystem designed to detect impurities in a gas;

FIG. 2 is a diagram of a flow arrangement in a system that can be usedto detect impurities, including sulfur-containing impurities, inbeverage grade gases;

FIG. 3 is a diagram of a system in a high pressure FTIR mode;

FIG. 4 is a diagram of a system in a low-pressure FTIR mode;

FIG. 5 of a diagram of a system in a sulfur mode; and

FIG. 6 is a plot showing research grade CO₂ drift test data obtained indeveloping a correction parameter for a back pressure regulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention now will be described more fully hereinafter withreference to the accompanying drawings, in which illustrativeembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items. Also, all conjunctions usedare to be understood in the most inclusive sense possible. Thus, theword “or” should be understood as having the definition of a logical“or” rather than that of a logical “exclusive or” unless the contextclearly necessitates otherwise. Further, the singular forms and thearticles “a”, “an” and “the” are intended to include the plural forms aswell, unless expressly stated otherwise. It will be further understoodthat the terms: includes, comprises, including and/or comprising, whenused in this specification, specify the presence of stated features,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. Further, it will be understood that when an element, includingcomponent or subsystem, is referred to and/or shown as being connectedor coupled to another element, it can be directly connected or coupledto the other element or intervening elements may be present.

It will be understood that although terms such as “first” and “second”are used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another element. Thus, an element discussed below could betermed a second element, and similarly, a second element may be termed afirst element without departing from the teachings of the presentinvention.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

The invention generally relates to techniques for detecting gasimpurities and is of particular interest in applications that requirevery high gas purity levels. In many of its aspects, the inventionrelates to detecting impurities, also referred to herein as“contaminants” in beverage grade gases, for instance contaminantsencountered in carbon dioxide (CO₂) gas and/or nitrogen gas (N₂).

CO₂ purity, for example, often depends on the CO₂ manufacturing process,plant purification methods, storage, transportation, point of useconditions and so forth. For the food and beverage sectors, for example,CO₂ is obtained via combustion, fermentation, from ammonia and hydrogenproduction. Both the preparation and the supply chain, often complex,that ultimately deliver the CO₂ to users can introduce impurities in theCO₂. Some of the contaminants particularly important to bottlers includeacetaldehyde, benzene, methanol, total sulfur content and totalhydrocarbons. Guidelines regarding CO₂ quality and best practices havebeen formulated by the International Society of Beverage Technologists®(ISBT), a group including beverage and CO₂ producers, distributors,providers of analytical and other related services and equipment.

Examples of impurities that can be identified and quantified practicingaspects of the invention include but are not limited to: SO₂, NH₃, CO,NO, NO₂, H₂O, HCN, CS₂, methanol, acetaldehyde, methane, totalhydrocarbons (methane, ethane, propane, pentane), benzene and totalaromatic hydrocarbons and others. In some implementations, the CO₂%present in a CO₂ sample gas also is determined. Detection is performedby FTIR gas analysis, employing methods and equipment described below.

In specific embodiments, the invention features a system comprisingseveral components, also referred to herein as “modules”. Embodiments ofthe system and its operation are described with reference to FIGS. 1through 5 .

As shown in the block diagram of FIG. 1 , for example, system 10includes: FTIR gas analyzer 12; oxidizer 14, typically including afurnace, for converting reduced sulfur to sulfur dioxide; multiplexer16, having multiple, i.e., two or more (e.g., 4 to 10) channels; climatecontrol module 18, used to maintain gas conditions suitable for accuratelow-level measurements; and an optional oxygen module 20, for situationsthat may involve low levels of oxygen gas (O₂) contamination.

In specific embodiments, sample 50 is selected in multiplexer 16 fromseveral input CO₂ samples, e.g., CO₂ feeds or streams #1 through #4 fora 4-channel multiplexer 16. In one implementation, these samples arederived from different beverage facility locations or process stages.

A first portion of sample 50, namely stream 50 a, is directed to FTIRgas analyzer 12 for an assessment of impurities present in the sample,including SO₂. For measuring reduced sulfur, a second portion of sample50, namely 50 b is directed to oxidizer 14, where non-SO₂sulfur-containing compounds are converted to SO₂. As a result, theoxidizer output 50 c will contain the sum of the original SO₂concentration present in the sample (already measured by the FTIR onstream 50 a) and the SO₂ concentration generated from reduced sulfur inoxidizer 14. Accordingly, the FTIR SO₂ measurements on output stream 50c will reflect the total sulfur concentration in the sample 50. Reducedsulfur content can be determined by subtracting the SO₂ value in sample50 a from the SO₂ value in sample 50 c.

For many applications, system 10 is under computer control illustratedby computer system 22, which can include additional elements such as,for instance, keyboard 24, touch screen 26, various connector, cables,electronic boards and/or panels, associated software, and so forth.Power module 28 supplies power (typically from an AC power source) tovarious components and/or their subcomponents.

In many cases, system 10 is designed to handle not only sample inputs(e.g., streams CO2#1 through CO2#4 in FIG. 1 ), but also other fluidstreams such as purge or calibration gases. In illustrative examples,system 10 is designed to handle a sample gas, e.g., a sample CO₂ gas; aspan or calibration gas (a gas containing known levels of impuritiessuch as propane, benzene, COS, in N₂ gas) that can be diluted into asample CO₂ to check the system calibration from time to time); anultra-high purity (UHP) purge gas, e.g., UHP N₂ gas; anoxygen-containing gas, e.g., clean dry air (CDA) for conductingoxidation reaction in oxidizer 14, for example. System 10 also canemploy an ultra-high purity (99.99+%) CO₂ gas, which serves to calibratethe system with respect to CO₂% determinations. In some implementations,this reference gas is supplied via multiplexer 16.

If needed or desired, suitable equipment can be included to furtherpurify gas feed streams, to filter particulates from the sample gas, andso forth. For instance, a CDA purifier (e.g., a model obtained fromParker Hannifin Corp.) can be added for situations in which CDA (ratherthan N₂) is used as zero and purge for the FTIR instrument. A carbonfilter or another suitable trap capable of removing contaminants presentin the CO₂ sample gas can generate purified CO₂ gas, which then can beused as the reference CO₂, in the FTIR analyzer, for example.

The various gases are provided at specified conditions, from suitablesources, entering and exiting the system or modules thereof at variousinlets and outlets. Within the system, fluid flow is directed throughconduits (lines) that can be made of plastic or another suitablematerial.

Components and illustrative configurations of system 10 are furtherdescribed with reference to FIGS. 2 through 5 .

Shown in FIG. 2 , for example, is one arrangement that can be employedto support the flow of gases not only in and out of the system but alsofrom one component to another, with lines connecting inlets and outletsspecific to a module and/or gas.

In the multiplexer 16, a sample stream, e.g., sample 50, is selectedfrom various samples (streams CO₂-1 through CO₂-9) by a selector(multi-position) valve V5. Sample 50 enters the FTIR spectrometer atinlet 11 and exits the FTIR at outlet 13. Oxidizer 14 receives thesample stream 50 (which can be premixed with an oxidizing gas, such asCDA) at inlet 15, while the processed stream leaves the oxidizer 14 atoutlet 17.

In more detail, sample 50 leaves multiplexer 16 at sample out 31, isdirected toward the FTIR gas analyzer 12 and enters the FTIR module atsample inlet 11. Oxidizer outlet 33 references the CO₂ sample mixed withO₂ (CDA) leaving the multiplexer module 16 in the direction of theoxidizer module 14 (entering the oxidizer at inlet 15) to generate SO₂.Oxidizer inlet 35 indicates the processed sample coming back (fromoxidizer outlet 17) to the multiplexer module 16, to be subsequentlysent to the FTIR gas analyzer 12 for measurement.

N₂ gas 54 a, at an illustrative pressure of in the range of from about65 psig (˜0.448 mega Pascal (MPa) or 79.7 psi absolute (psia),corresponding to ˜0.550 mega Pascal) to 80 psig (˜0.552 MPa) or 94.7psia (0.653 MPa)) can be obtained from a suitable source, a gas bottleor cylinder, for instance. It is directed to valve V5 and from there itcan be delivered to the gas cell 300 in the FTIR instrument (along thelines supporting sample flow), to purge all potential contaminants,e.g., atmospheric species such as CO₂, CH₄, CO, H₂O). N₂ gas also can beused to purge the optics in the FTIR instrument, as can CDA, in somecases. FIG. 2 shows the latter entering the multiplexer module at purgeinlet 53; this same inlet can be used to introduce purge N₂ gas. In oneexample, N₂ purge gas that is used to purge the FTIR optics (stream 54b) leaves the multiplexer module 16 at FTIR purge out 41, enters theFTIR module at FTIR purge inlet 43 and exits the FTIR module at FTIRpurge outlet 45. For purging the FTIR optics, N₂ gas can be derived fromthe same N₂ source as that providing stream 54 a. However, the purge N₂is not directed to V5 but rather towards purge inlet 53.

If present, the oxygen module receives the sample stream 50, the O₂ zerogas, typically high purity CO₂ gas 58 (obtained, for instance, viachannel 12 of multiplexer 16), and O₂ span gas 62, at inlets 19, 21 and23, respectively.

Other gas streams that can be employed include CDA gas 56, supplied froma suitable source 61 at a pressure of about 65 psig (0.448 MPa or 79.7psia (˜0.550 MPa)) to about 80 psig (˜0.552 MPa or (94.7 psia (˜0.653MPa)), for example, and calibration (span) gas 52 (containing impuritiessuch as propane, benzene, COS), supplied from source 63 at a pressure ofabout 80 to about 90 psig.

Port 47 is the sample in port from which sample can be delivered to theO₂ sensor (if present) and/or to a mass flow control (MFC 84 in FIGS.3-5 , for example) for delivery to the FTIR gas analyzer 12 or oxidizermodule 14. CDA gas 56 can enter the multiplexer module at CDA inlet 51and CDA purge gas at inlet 53 (for situations in which CDA (instead ofN₂) is employed as a purge gas). Calibration gas 52 enters themultiplexer 16 at inlet 55.

Gases can exit the system via suitable vents. Vent 42, for example, canbe used to exhaust gas from the FTIR gas analyzer 12. In specificimplementations, FTIR 12 is provided with back pressure regulator 40(for maintaining a defined pressure upstream of its location) and valveV1 (for bypassing back pressure regulator 40 and allowing gas to proceedto vent 42). If present, oxygen module 20 can be exhausted via vent 44.In specific implementations, the vents are configured with safetyconsiderations in mind, to exhaust CO₂ to a safe location, away from theanalyzer, for example.

Optionally, system 10 can include a sensory output 46 (FIGS. 3-5 ) forodor assessment by a qualified technician.

Other devices for monitoring or controlling the flow of gases throughsystem 10 are shown in FIGS. 3 through 5 . Among them are span(calibration) mass flow controller (MFC) 82 and CO₂ main MFC 84.Pressure sensors P1, P2, P3, and P4 allow the incoming pressureregulators to be set to provide the correct pressures to the system andare monitored to ensure having the proper pressure at each point withinthe system. PS elements are switches used to confirm the pressure aftera gas has moved through a pressure regulator and can alarm if a pressureis below a needed pressure value. Rotameters can be used in some cases.

In addition to the selector valve V5, system 10 can include variousvalves, such as V1, V2, V3, for blocking or allowing gas flow. In someembodiments, system 10 employs a rotating valve V4 (e.g., a VALCO, 2position, 4 port, valve), further described below.

As shown in FIGS. 3 through 5 , various gases enter or exit system 10 atinterface panel 93. A sequence in which various gases are provided, andgas parameters relative to system components can be as follows.

As a first step, N₂ is passed through to zero the entire system. Withonly N₂ present, the pressure at the FTIR instrument can be at near 60psig (˜0.414 MPa or 74.7 psia (0.515 MPa)). This is followed bymeasuring the CO₂ high purity gas 58 (CO₂-ref, line 10 at valve V5), toset the calibration on the CO₂% reading. In some approaches, ultra-highpurity CO₂ can be supplied in a separate cylinder of known-pure gas orcan be the sample CO₂ that has been purified on-site (passed throughcarbon filters, for example).

A next step involves blending the span gas 52 into the CO₂-Ref gas(stream 58) to check the span on impurity gases like propane, benzeneand reduced sulfur (COS). Other gases can be added to this mixture ifdesired or requested for a particular analysis. Corrections for theamount of compounds present can be made before or after injection of thecalibration gas. Typically, the span gas is blended upstream of the mainCO₂ MFC 84 to prevent oscillation in the span gas MFC 82 when switchingfrom a high to a low downstream pressure. This results in a relativelyconstant pressure (about 75 psig (˜0.517 MPa or 89.7 psia (0.618 MPa))to about 80 psig (˜0.552 MPa or 94.7 psia (0.653 MPa)) at the lowpressure (output) side of the span MFC 82. In a typical operation, thepressure on the span MFC is normally 5 psig (˜0.034 MPa or 19.7 psia(˜0.136 MPa) to 10 psig (˜0.069 MPa) or 24.7 psia (˜0.170 MPa)) higherto prevent back flow. An arrangement in which V3 is opened (for addinggases) and closed (to prevent sample contamination from calibration gas52 and to prevent it from bleeding out over time) allows the user toleave the calibration gas line pressurized. To prevent loss ofcalibration gas, the calibration gas can be turned on only when needingto perform this step.

To measure reduced sulfur, the back pressure regulator 40 is bypassed byopening V1 and the FTIR gas cell pressure is reduced such as to lessthan 2 atmospheres (atm) (less than ˜0.203 MPa) or specifically, e.g.,to 0 psig or 1 atm (˜0.101 MPa). A sample spectrum is collected on thesample CO₂ at this point, to be used as an interference spectrum in thequantification for calculating the reduced sulfur as SO₂. Theinterference spectrum reflects the SO₂ initially present in the samplestream 50 a as SO₂. The flows are then switched (by the rotating valveV4, and opening V2 as further described below) so that CDA is added tothe sample CO₂ forming a stream 50 b that is directed to oxidizer 14 forthe conversion to SO₂. Processed stream 50 c exits oxidizer 14 and isdirected to the FTIR gas analyzer 12. Once the processed stream from theoxidizer has stabilized, an infrared spectrum is obtained to measure theSO₂ in stream 50 c and, using the interference spectrum, determine thetotal reduced sulfur present.

Turning to a more detailed description of the modules included in system10, FTIR gas analyzer 12 can be constructed using principles describedin U.S. Pat. Nos. 9,606,088, and 10,054,486, to Spartz et al., bothdocuments being incorporated herein in their entirety by this reference.

Typically, FTIR spectrometer 12 employs a “gas cell” 300 (also referredto herein as a “sample cell”) made from a suitable material, forinstance welded stainless steel. In one implementation the cell haswelded stainless lines and the cell is aluminum with a nickel plating.The gas cell can be maintained at a desired temperature (e.g., roomtemperature or higher, about 35° C., for instance).

The FTIR spectrometer 12 has an interferometer such as a Michelsoninterferometer and a polychromatic infrared source. In a typicalconfiguration, light from the source is collimated and directed to abeam splitter of the interferometer that divides the light between afixed mirror in one arm and moving mirror in the other. The light fromthe arms is mixed back at the beam splitter and then directed to thesample cell and then to the detector. In a current configuration, bothmirrors move to reduce the distance required to generate 4 cm-1 spectra.One is moving towards the beam splitter while the other is moving away.Then it turns around and goes in the opposite direction. Thisconfiguration also uses corner cubes as the interferometer mirrorsinstead of flat mirrors found in a traditional Michelson design.

In many embodiments, the gas cell is fabricated to withstand pressuresabove atmospheric. In specific examples, the cell is designed forpressures such as 2, 3, 4, 5 or higher atmospheres (atm). If designed asa flow-through gas cell it can support flow rates such as, for instance,2 to 7 liters per minute or 2000 to 7000 sccm (standard cubic centimeterper minute) or higher. In fact, for the impurity measurement the flowrate is 7,000 sccm or even higher to speed up the turnover of the cell.A lower flow can be employed on the switch to low pressure, such as lessthan 3000 sccm, or, in one example, 1,725 sccm of CO₂ with 75 sccm ofair for a total flow of 1,800 sccm. This ensures full conversion of thesulfur to SO₂ and also that the reactor is not cooled with cold gas.Often, the source pressure is considerably higher than the pressure inthe gas cell and a pressure regulator can be included.

Increasing the pressure in gas cell 300 can improve sensitivity. Usinghigh pressures of CO₂ is feasible since CO₂ does not absorb strongly inthe IR spectral region in which the impurities of interest typicallyabsorb. Thus, high amounts of CO₂ in the cell are not expected tointerfere with the spectral features observed for the contaminants.Another gas that can be monitored for contamination is N₂. With noabsorption in the region of interest, even higher (above 5 atm, forexample) pressures of N₂ can be introduced in the cell, limited only bythe materials and construction of the cell.

Typically, the gas cell windows are BaF₂ or CaF₂ and for manyapplications they too are designed to handle pressures higher thanatmospheric, such as the pressures discussed above. Windows that are notwater sensitive and do not have high refractive indices are preferred.

Some embodiments utilize a multiple reflection gas cell, such as, forinstance, a White cell. Traditional White cells arrangements includethree spherical concave mirrors having the same radius of curvature.Modified White cells or other multiple path designs, e.g., Herriottcells, Pfund cells, cavity-ring down cells, and integrating spheres,also can be employed, as described by Spartz et al. in U.S. Pat. Nos.9,606,088 and 10,054,486. By increasing the path length traveled,multiple-pass arrangements can measure low concentration components ordetect weak absorption spectral features without increasing the physicallength or volume of the sample cell itself.

Longer path lengths can be used in combination with higher reflectivecoatings such as enhanced silver.

In one non-limiting example, the gas cell uses non-spherical concavemirrors to improve image quality and optical throughput. The mirrors arecut onto a single metal or a glass blank, providing a fixed path length;the mirrors can be the solid end caps of the gas cell, allowing forsmaller sample cells that are easier to align. Such a “White” cell has atypical transmission of about 50%. Switching to enhanced silver mirrorscan result in a transmission increase of up to 70% (meaning that about40% more light will reach the detector for potentially 40% lower MDLs).For an exemplary White type cell with a volume of about 500 mL, usinggold mirrors can produce a path length of about 9.86 meters (m), whileenhanced silver mirrors can result in a path lengths of 10 m or longer.

A FTIR with a multiple pass White cell can have a large dynamic range.In one illustration, the dynamic range, namely the range from the limitof detection to the maximum signal that can be measured, is 1.0 ppbimpurities to % CO₂ readings. The accuracy can be <+/−5% and theprecision can be <+/−2% of reading. Typically, the linearity is betterthan 1%. In many cases, the CO₂ measurement are 100% +/−0.03%. The MDLsfor impurities can range from 1 to 50 ppb, depending on the impurity.For instance, the MDL for reduced sulfur can be about 10 ppb.

For some applications, the sample cell can be a lightpipe flow-throughsample cell.

The FTIR instrument can be provided with several options such as, forexample: off or no flow; zero (using N₂ or high quality CDA); spanmixture for calibration checking; and sample. The hardware, datacollection, processing and reporting can be all handled together, e.g.,using the MAX-Acquisition™ software package.

When the instrument is in the “off” sample selection, no gas enters theinstrument. In the zero sample selection, purge gas (nitrogen or CDA, insome cases) sweeps through the gas cells. After purging the instrument(e.g., for 10-15 minutes) a new background can be taken, to obtain aclean reference single beam spectrum for comparison to new sample data.For the sample mode, purge gas is stopped and a current single beammeasurement is obtained and compared to the reference single beam,generating an absorbance spectrum.

For specific applications such as measuring contaminants in a CO₂ gassample, FTIR spectrometer 12 can be provided with calibrations for alarge number of contaminant species. Others can be added as needed.

While in some cases all that is needed is measuring impurities, system10 can also measure the concentration of CO₂ present in a CO₂ bulk orsample gas, typically expressed as CO₂%. This determination can beperformed using CO₂ gas 58, which can be 99.999% pure, as a span gas toconduct a calibration operation. In the embodiments of FIGS. 2-5 , theultra-high purity CO₂ gas can be accessed by positioning valve V5 influid communication with the CO2-ref line. In one example, thecalibration step is conducted by setting the flow rate of the main MFC84 at 5 to 7 liter per minute (LPM) and the span MFC 82 to zero. Closingvalve V3 prevents bleeding through the span MFC 82.

The spectrum of the ultrahigh purity CO₂ can then be compared with thecollected spectra of the sample gas, the results indicating whether thesample gas is low quality, containing, for example, significant amountsof N₂ and/or O₂.

A high purity CO₂ calibration spectrum also can play a role in theimpurity analysis. In one example, this high purity CO₂ spectrum resideson the computer, e.g., in a calibration database, and is utilized toremove the overlapping CO₂ spectral features while quantifyingimpurities. It was found that a calibration spectrum that matchesclosely the sample spectrum will address possible biases for compoundssuch as CO, N₂O, and/or benzene.

In some implementations, functionality is added to create an instrumentvalidation stream internal to the instrument, to allow measurements of aknown concentration of gases via the FTIR and thus ensure that theinstrument is working properly.

FTIR spectrometer 12 can be, for instance, a MAX-iR FTIR Gas Analyzer,available from Max Analytical Technologies (East Windsor, Conn.), now apart of Thermo Fisher Scientific. Some illustrative features include:real-time gas analysis (1 sec-1 min response); 1-32 cm⁻¹ resolution FTIRspectrometer; VCSEL laser diode (long life); SiC IR source (long life);non-hygroscopic (no purge required); precision temperature and pressuresensors. The apparatus can be provided with a 5 U-19 inch standard rack;a 10 m high throughput multipass gas cell; and integrated analysismethods.

As already mentioned, some aspects of the invention relate to variousfeatures or techniques employed to circumvent the UV fluorescencemeasurements and apparatus taught in U.S. Pat. Nos. 10,408,746 B2 and10,761,018 B2.

An important consideration to reach this goal is the selection of a FTIRdetector that is sensitive over a broad IR frequency range. In specificexamples, the detector is sensitive over a range covering spectralfeatures associated with non-sulfur impurities such as acetaldehyde,aromatics, hydrocarbons, methanol, ammonia, etc., as well as thespectral features associated with sulfur dioxide (SO₂), in its typicallyIR region of 1,300-1,400 cm⁻¹. A deuterated triglycine sulfate (DTGS)detector, for example, allows for measurements in the entire mid-IRspectrum. DTGS detectors are thermal detectors for which the signal isgenerated in response to the change in temperature caused by absorptionof the IR radiation. The response time of these detectors can be lessthan 1 millisecond (ms), allowing them to follow changes occurring atrates up to several kilohertz.

As noted in U.S. Pat. Nos. 10,408,746 and 10,761,018, DTGS detectors maypresent some disadvantages when compared with mercury cadmium telluride(HgCdTe or MCT) detectors. Nevertheless, it was discovered that, whenintegrated within system 10, DTGS detectors can measure the very lowcontaminant levels (including non-sulfur as well as SO₂ species), asrequired for analyzing beverage grade gases.

The FTIR instrument can be operated at 4 cm⁻¹ resolution at about 5second data collection with Cosine Apodization. A rolling average of 1to 5 minutes can be used with display updates at 5 to 6 secondintervals. A lower resolution, such as 8 cm⁻¹ resolution, might providesomewhat better MDLs for some of the compounds of interest like benzeneand the total aromatics. Using too low a resolution may result in lossesin the capability of measuring some of the gases having narrowabsorption bands. One illustrative example employs batch analysis. Inthis approach, measurements are obtained over a period of time, the dataare averaged, and the results reported.

Configurations can be designed to obtain 1 ppb MDL for benzene. Allother compounds can be detected with improvements of 100 to 1000 overthe values required by the ISBT.

In practice, benzene often can be used as a surrogate for all aromaticimpurities. Since all the aromatic impurities absorb in the samespectral region (3000-3200 cm⁻¹) and can be measured as a group bymeasuring just one, it is possible to quantify for benzene and report asbenzene and total aromatic hydrocarbon content. Preferably, linearregression is used to determine the area under the spectral plot in thisaromatic region. In general, these aromatic compounds produced about thesame signal per mole.

Light VOCs such as methane, ethane, propane and pentane can be measuredfor the aliphatic hydrocarbons (most are in the 2750-3000 cm⁻¹ region).In some situations, the separation power of chromatography can becombined with the quantification power of absorption spectroscopy, asdescribed, for example by U.S. Pat. Nos. 9,606,088 and 10,054,486. Suchan approach can be particularly useful in detecting VOCs, for example.

Absorbance spectra can be analyzed via classical least squares (CLS)fitting to determine the level of contaminants present in the CO₂stream. In typical implementations, both the background and the sampleare analyzed at greater than 2 atm, and preferably greater than 3 or 4atm and possibly 5 atm or higher to eliminate differences due topressure. Preferably, calibrations for all contaminant gases arecollected at the analysis conditions.

Techniques described herein can provide information on total sulfurimpurities, sulfur dioxide (SO₂) as well as reduced sulfur. As notedabove, FTIR 12 can register the presence of SO₂ directly. Obtaining dataon non-SO₂ sulfur species (also referred to herein as “reduced” sulfur)relies on oxidizer (module) 14, which includes a furnace or anothersuitable apparatus for converting the reduced sulfur present incontaminants such as H₂S, CH₃SH, CS₂, COS (carbonyl sulfide), (CH₃)₂S,(CH₃)₂S₂, and/or other compounds, to SO₂.

The conversion (oxidation) reaction can be conducted using an oxidizingmaterial such as oxygen, oxygen-enriched air or air, typically CDA. Inone example, sample CO₂ is mixed with CDA (e.g., stream 56 in FIGS. 2-5) prior to entering oxidizer 14. One or both streams (i.e., CDA and/orthe CO₂ gas sample) can be preheated to a suitable temperature, tofacilitate the oxidation reaction. Another approach involves heating thestream resulting from the combination of the sample stream and CDA 56.

Suitable means for heating the desired gas or gases include heatexchangers, heating tape, heating jackets, ovens, Peltier heaters,cartridge heaters, and so forth. It is also possible to preheat a streamobtained by combining (mixing) CDA and CO₂. In many cases, the gas(es)entering the oxidizer will be at a temperature ranging from roomtemperature to a temperature that is equal to or lower than thetemperature of the furnace.

To reduce the cost and complexity of the system, the air or O₂ additioncan be controlled by a rotameter such as rotameter 71 in FIGS. 3-5 . Thetotal added flow (or sample dilution) can be monitored by measuring thechange in the % CO₂. As a result, an accurate measured flow of air andO₂ is not required.

One example of an oxidizing apparatus that can be employed is describedin U.S. Pat. Nos. 10,408,746 B2 and 10,761,018 B2. Such an apparatusincludes a furnace where, in the presence of heat and an oxidizing gas,sulfur-containing compounds other than SO₂ yield SO₂. Typically, the SO₂output from the furnace and, more generally from the oxidizer module 14,will include (i) SO₂ already present as such in the sample gas, and (ii)SO₂ generated through the conversion taking place in the furnace. Asdescribed in the '746 and '018 patents, oxidizer 14 can be provided withpermeation devices, restrictors, and/or other elements. In someimplementations, oxidizer 14 is a commercial apparatus or a unitthereof.

Oxidizer 14 can have three options: (1) zero, (2) span, and (3) sample,and can be controlled by its own independent software. The correct flowrates can be set via regulators that can be internal to the particularsystem used. Once thermally stable, the oxidizer is placed in the zeromode, where the sample CO₂ is scrubbed via a charcoal filter to removecontaminants and then analyzed. The instrument zero value is taken viaan internal reference zero function.

Next, the instrument is placed into span mode, employing, for example, aspan cylinder to introduce a known level of COS to the bulk CO₂. Aninternal auto span function can correct the calibration curve to thespan response.

After these two calibration steps are conducted, the instrument isplaced into sample mode where any sulfur species is converted to SO₂ andthen reported via the FTIR spectrometer. Typically, the calibration andmeasurement steps are performed at less than 2 atm such as ambientpressure (1 atm).

One suitable protocol involves obtaining a FTIR spectrum of sample 50 aat low pressure to get an interference spectrum of the sample. The flowis changed (at valve V4) so that sample 50 b, with O₂ added (using CDAstream 56) is directed to furnace 27 of oxidizer 14. The processedstream exits as stream 50 c and is analyzed in the FTIR instrument. Thespectrum of gas 50 a can be used in the analysis, so that the measuredresult would simply reflect the newly generated SO₂ (produced in theconversion), which represents the reduced sulfur gases.

Performing the measurements described above involves directing gases to:(a) oxidizer 14 and then to the FTIR instrument, or (b) just to the FTIRspectrometer 12, bypassing the oxidizer. In one arrangement, system 10includes valve V4 (e.g., a VALCO, 2 position, 4 port, valve), which canbe used in two configurations. MFC 84 can control gas delivery to V4 ata flow rate of, for instance, 2-5 liters per minute (LPM).

In configuration A (shown in FIGS. 2 and 3 ), ports 1 and 2 are in fluidcommunication with each other, as are ports 3 and 4. Configuration A canbe used to direct sample gas (labeled, as stream 50 a, typically at apressure higher than atmospheric such as greater than 2 atm (˜0.203MPa), and preferably greater than 3 (˜0.304 MPa) or 4 atm (˜0.405 MPa)and possibly 5 atm (˜0.507 MPa) or higher, such as 60 psig (˜0.414 MPaor 74.7 psia (0.515 MPa)), for instance, to FTIR spectrometer 12. V4 canbe rotated to configuration B (in which ports 1 and 4 are in fluidcommunication, as are ports 2 and 3. Configuration B, shown, forexample, in FIG. 4 , directs flow to furnace 27 of oxidizer 14 andallows processed gas exiting the oxidizer to enter the FTIR gas analyzer12. In one implementation, as sample gas flows towards the oxidationchamber (e.g., furnace 27 in FIG. 4 ), a small amount of air or O₂(e.g., CDA stream 56) is added (via rotameter 71) to the sample stream.Reduced sulfur present in stream 50 b is converted to SO₂ and stream 50c (processed gas), typically at atmospheric pressure, leaves theoxidizer and is analyzed using FTIR spectrometer 12.

System 10 can be operated in several modes: a high pressure FTIR mode(illustrated in FIG. 3 ), a low pressure FTIR mode (illustrated in FIG.4 ), and a sulfur mode (illustrated in FIG. 5 .

Specifically, the high pressure FTIR mode (FIG. 3 ) involves V4 inconfiguration A, V1 in the closed (off) position so that gas cell 300can be operated at pressures higher than atmospheric. V2 is closed(off), blocking CDA stream 56. Also closed (off) is V3, blocking theflow of calibration (span) gas 52. The FTIR data obtained in this modereflects the SO₂ amounts initially present in the sample gas 50 a.

For the low pressure FTIR mode of FIG. 4 , V4 is in the A configuration(positions 1-2 and positions 3-4 in fluid communication), V1 is open,while V2 and V3 are closed. In this arrangement, pressure can bereleased by letting accumulated gas (e.g., sample 50 a) from the samplecell 300 to proceed towards vent 42, reducing the back pressure in thesample cell to ambient pressure. The low pressure mode can be employedto prepare the FTIR gas analyzer 12 for conducting the reduced sulfurmeasurements at atmospheric pressure.

In the sulfur mode of FIG. 5 , V4 is rotated to the B configuration withfluid communication for positions 1-4 and 2-3, allowing the samplestream access toward furnace 27 of oxidizer 14 and flow of the processedstream 50 c from the oxidizer to FTIR 12. With V2 in the open position,CDA 56 is allowed to blend with the sample stream, forming stream 50 b,directed to furnace 27. V3 is in the closed position, blocking the flowof calibration gas 52. This mode bypasses the FTIR spectrometer untilafter the sample gas has been processed in oxidizer 14. At that stage,the FTIR measurements on the output stream from oxidizer 14, namelystream 50 c, are conducted in the low pressure FTIR mode (V1 is in theopen position). As already noted, stream 50 c reflects the total amountof sulfur as the sum of the SO₂ initially present in sample 50 a and theSO₂ produced by the oxidation reactions taking place in oxidizer 14. Ifthe FTIR spectrum of sample 50 a is used as an interference spectrum,the FTIR analysis of the 50 c stream can reflect the amount(concentration) of the reduced sulfur present in the sample.

In many embodiments of the invention, various gas feeds (streams) areintroduced and analyzed, in a desired sequence, using multiplexer 16.This component (module) allows a gas sample to be selected for analysisfrom multiple (two or more) gas samples. The output data are generatedby the FTIR instrument in a sequence corresponding to the sequence inwhich different gas samples are being introduced.

Two and preferably more than two (e.g., 4 to 10, in specificimplementations) sample channels can be employed. Multiplexer 16 alsocan be used to introduce ultra-high purity CO₂ gas 58 (shown as CO₂ refin FIGS. 2 through 5 ).

A valve arrangement opens or closes fluid communication for selectivelyallowing (or blocking) the flow of a specified gas. In one embodiment,multiplexer 16 employs a selector (multi-position) valve V5 (from ValcoInstruments Co. Inc.). In FIGS. 2 through 5 , V5 has 12 positions forintroducing N₂, CO₂-ref #10 and samples CO₂ #1 through #9. Anotherimplementation utilizes a 6-position Valco selector valve. Selector 16of FIG. 1 has 4 inputs, specifically CO₂#1 through CO₂#4). Otherarrangements can be employed to introduce gas feeds in a desiredsequence.

To meet optimal conditions for obtaining the measurements describedherein, system 10 is provided with climate control module 18 whichinvolves sensors, regulators, and/or other means for controlling thephysical properties of a gas stream passing through the system. Forinstance, in addition to measuring trace impurities, system 10 also canbe used to determine percent CO₂ in a bulk or ultrapure CO₂ stream. Adesired accuracy can be ±0.02% in a gas at near 100.0% CO₂. To calculateCO₂%±0.02% accuracy at near 100.0%, however, requires precisetemperature and pressure, along with precise FTIR spectra.

In specific implementations, gas cell temperatures are maintained stableto ˜0.03° C. This is found to involve measuring the actual gastemperature rather than simply the gas cell temperature. To meet thisrequirement, temperature sensors measure temperatures to ˜0.03° C. andare configured and disposed to measure the temperature of the gas itselfrather than that of the gas cell. Also preferred are sensors that canhandle high pressures without leaking. Gas stream temperatures can becontrolled using heat exchangers, heating tape, heating jackets, ovens,cooling chambers, Peltier heaters, cartridge heaters, and so forth.

In many cases, desired pressures need to be constant at 5.00 atm. In oneexample, the gas cell pressure is maintained at 5.00 atm≤±0.01 atm.Accordingly, preferred pressure gauges are designed for accurate andprecise measurements to ±0.0005 atm at 5.00 atm. In some embodiments, apressure sensor is installed on the gas cell to ensure that the exactpressure within the cell is known (not just the pressure on the gas linegoing into and out of the cell).

Another approach compensates for pressure drifts, such as might beencountered in a back pressure regulator (e.g., pressure regulator 40),by using a correction of the CO₂ reading. To illustrate, a Swagelok backpressure regulator that has some drift at a pressure of 5 atm is modeledand the data are corrected for any change in pressure from the reference5.00 atm. A pressure correction equation was developed using researchgrade CO₂ drift data, which were collected in a well-controlledenvironment. The data are shown in FIG. 6 . A linear regression wasdeveloped to model the variance in the CO₂ concentration from 100.000%(ΔCO₂) as a function of the variance in measured pressure from 5 atm(ΔP). From this linear regression, the pressure correction equation wasderived to report an adjusted (corrected) CO₂ concentration. An equationthat can be used is:

CO₂ corrected=(Measured CO2+(15,500×(5.00−Measured pressure)))*CF

where CF is a correction factor specific to each instrument forcorrecting for small errors in the temperature and pressure readings.

System 10 can include optional oxygen module 20, useful for determiningoxygen levels in a sample CO₂ gas. If present, oxygen module 20 employsa detector or analyzer that relies, for example, on one or more oxygensensors, such as: electrochemical oxygen sensors, zirconia oxygensensors, optical oxygen sensors, Clark oxygen sensors, infrared oxygensensors, electro galvanic sensors, ultrasonic oxygen sensors, and laseroxygen sensors. In one illustration, the gas sensor is designed tomeasure oxygen levels at the parts per million or parts per billionlevel. Many sensors can be further configured to provide a feedbackmechanism for controlling the oxygen level in the gas stream. In many ofthe embodiments described herein, O₂ levels are determined by a sensoror sensors that operate independently of FTIR spectrometer 12.

As discussed with reference to FIG. 2 , oxygen analyzer 20 can have azero mode (performing measurements on gas stream 58, a high purity CO₂gas that can be supplied, for instance, via multiplexer 16), a span mode(using oxygen span gas 62) and a sample mode using, for instance any ofthe samples CO₂#1 through #9 selected via multiplexer 16. In FIGS. 3through 5 , the sample gas is labeled 50 d. In specific implementations,the span gas is run at a single level; it is not diluted with CO₂ forany gas delivered from the multiplexer.

Illustrative pressures can be 20 psig (˜0.138 MPa or 34.7 psia (˜0239MPa)) for the O₂ span gas, 75 psig (˜0.517 MPa or 89.7 psia (0.618 MPa))for the zero calibration gas and 75 psig (˜0.517 MPa or 89.7 psia (0.618MPa)) for the sample gas. The gas contents are exhausted from optionaloxygen module 20 at vent 44.

Gas inputs, flows, calibrations, sample analysis and/or other operationscan be automated, as can be switching between the various channels,pressures, flow pathways and so forth. Pressure controls and valves thatcan rely on actuators (e.g., actuators 88 in FIGS. 3-5 ) as well asother components of system 10 can be controlled by a controller,computer system 22, for instance.

In many embodiments, functions performed by climate control module 18 orcomponents thereof are automated. In one illustration, climate controlmodule 18 includes two Peltier heating/cooling systems, the temperatureset points for each being controlled by computer system 22.

Other features that can be provided include touch screen technology,programmable logic controllers (PLC), remote pressure and/or mass flowcontrollers (MFC) to control one or more gas streams, multiple (e.g., 4or more), automated sample channels, to name a few. Ethernet, Modbusand/or other means can be used for data communication and remotecontrol. Further implementations utilize wireless communications, based,for instance, on existing or developing technologies.

Several or all functions can be centralized. In one implementationsystem 10 employs MAX-Acquisition™ to control and report everything.

Other information can be provided via Modbus or another suitableprotocol. Some implementations allow users to identify maximum allowableconcentrations for all gases, warning percentages for a list of gases,and the amount of time to average data. For instance, after pulling inand averaging results, concentrations of contaminants are displayed in adesired format. In one example, a graphical representation of the datais shown via a bar chart on the right. Both the concentration and itscorresponding bar can change between green, yellow, and red, forinstance, based upon the user settings for maximum concentrations andwarning percentages.

Conducting a desired gas analysis relies on operating the variouscomponents, along with the appropriate hardware and software algorithms.Setting the various functional states to obtain each measurement isfurther described in the illustrative example below.

EXAMPLE

This example presents a protocol that can be implemented on system 10 asdescribed with reference to FIGS. 1-5 .

As a first step, a purge gas, e.g., ultra high purity (UHP) nitrogen gassuch as stream 54 a in FIG. 2 , typically having a nitrogen gasconcentration of 99.999% with negligible impurities) is provided (viamultiplexer 16, for example) and directed to FTIR spectrometer 12 tocollect a background spectrum. The purge flow pathway 91 (FIGS. 3-5 ),can include a pressure switch PS (15 to 80 psig (0.103 to ˜0.552 MPa) or29.7-94.7 psia (˜0.205 to ˜0.653 MPa) and rotameter 74 and typicallysupports N₂ purging. In some cases, the purge gas can be CDA (gas 56 inFIGS. 3-5 ), however.

CO₂ gas 58 (provided, for instance as CO₂-ref from channel 12 ofmultiplexer 16) is ultra-pure and can be utilized to obtain a baselineFTIR spectrum. In specific examples, the reference CO₂ in the FTIRsample cell 300 is maintained at a pressure higher than atmospheric,e.g., a pressure that is the same or close to the pressure of the sampleCO₂ in the gas cell 300, e.g., about 5 atm. IR features of the referenceCO₂ can be subtracted from the IR features of the sample CO₂, resultingin spectral features that can be used to identify impurities in thesample of interest.

If the reference CO₂ has water, the spectral signatures of thisinterference can be subtracted from the reference spectra for the pureCO₂. Care to avoid negative concentrations is highly recommended.Specifically, the reference spectrum is reviewed for any positivebiases.

Alternatively, or additionally, calibrations for high purity CO₂ can beresident on the computer system 22 or accessible to it. For instance,CO₂ calibration spectra can be available in a database.

Sample CO₂ can originate from one of several (i.e., two or more)locations from a facility or plant. In one example, CO₂ samples areprovided from 4 or more locations at a plant handling beverage gradeCO₂. The selected sample gas feed 50 (flowing directly from one of thechannels in multiplexer 16) is introduced to the FTIR instrument asstream 50 a, to obtain the impurity and the % CO₂ measurements.Typically, these measurements are performed at a temperature above roomtemperature, 35° C. in many cases, and above atmospheric pressures (5.0atm, for example). Pressures above atmospheric are found to decreasedetection limits. Pressurizing the analysis (gas) cell to 5.0 atm, forinstance, can result in minimum detection limits (MDL) in the low ppbrange for contaminant gases such as, for instance, CO, NO_(x), benzene,acetaldehyde, acetone and SO₂. This pressure can be controlled by backpressure regulator 40, which can be set prior to the collection of thebackground or sample gas.

As a functional consideration, when the system is switched from N₂ toCO₂, the pressure is preferably modulated between 1 and 5 atm two ormore times, for 5 cycles, for example, with CO₂ flowing to the gas cell300 to remove trace N₂ from the gas cell (dead legs and trapped N₂).This is performed so that the % CO₂ reading is not affected. (Theconverse is also true: if a new N₂ background is required, N₂ at highand low pressure is introduced to clear any residual CO₂).

After the measurements on sample gas 50 a are performed, the FTIR gascell pressure is reduced by opening valve V1 (which bypasses the backpressure device 40), so that total sulfur measurements can be made at˜1.0 atm. Equilibration of CO₂ at 1 atm can take a minute or less, 30seconds, for example.

Once the pressure is stable, an infrared spectrum is collected to zeroout all interferences (CO₂, water other gases present in the bulk) tothe SO₂ generated from the oxidizer 14. This step can take 1 minute orless, e.g., 30 seconds for instance.

After the interference spectrum is collected, V4 is rotated to send thesample gas, e.g., bulk CO₂ (or other bulk gas) to the ˜1,000° C.oxidizer. As the sample gas flows towards the oxidation chamber a smallamount of air or O₂ (e.g., CDA 56) is added to the stream (using device71, e.g., a rotameter) to allow for the oxidation of reduced sulfur toSO₂. The resulting stream, 50 b, is processed to convert non-SO₂sulfur-containing species to SO₂, to produce stream 50 c.

Switching valve V4 to sulfur mode allows for FTIR measurements of theSO₂ present in the gas stream 50 c. Total reduced sulfur can be obtainedby subtracting the measurement of the SO₂ initially present in thesample gas as SO₂ (as measured directly by the FTIR on sample stream 50a) from the measurements obtained on the gas exiting the oxidizer(stream 50 c). In one illustration, the SO₂ analysis requires a minuteor so.

To start the next measurement the process is conducted in reverse. Theair or O₂ source is turned off at valve V2. V4 is returned to itsinitial position, V1 is closed, and the sample is switched to the nextchannel on multiplexer 16.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A system for analyzing a beverage grade gas, thesystem comprising: a multiplexer for selecting a gas sample frommultiple gas samples; a spectrometer including a gas cell for detectingan absorbance spectrum of gas in the gas cell; an oxidizing furnace forconverting reduced sulfur present in the gas sample to SO₂; anarrangement including a device for directing the gas sample to thespectrometer or to the oxidizing furnace; and a computer module foroperating the multiplexer, the oxidizing furnace and/or the device fordirecting the gas to the spectrometer or to the oxidizing furnace. 2.The system of claim 1, wherein the gas cell is a multiple path type celland/or wherein the gas cell is configured for pressures higher thanatmospheric.
 3. The system of claim 1, wherein the spectrometer isprovided with a FTIR detector for measuring SO₂ and non-sulfurimpurities in the sample gas.
 4. The system of claim 1, furthercomprising conduits, valves, an oxygen analyzer, and/or controls forpressure, temperature and/or flow rates.
 5. The system of claim 1,wherein the system does not include a UV fluorescence apparatus.
 6. Thesystem of claim 1, wherein the multiplexer is configured to receive gasfeeds from multiple plant locations.
 7. The system of claim 1, whereinthe computer module further comprises one or more functions forcollecting, analyzing and/or reporting data from the spectrometer. 8.The system of claim 1, wherein the computer module comprises connectionsto one or more of the multiplexer, the spectrometer and the oxidizingfurnace and/or is configured for local area network communications. 9.The system of claim 1, further comprising software for collecting,analyzing and/or reporting data.
 10. The system of claim 1, furthercomprising calibration information for pure CO₂ and/or one or moreimpurity for the spectrometer.
 11. A method for analyzing a beveragegrade gas, the method comprising: (a) selecting a gas sample frommultiple gas samples; (b) directing a first portion of the gas sample toa gas cell; (c) measuring impurities present in the gas cell with aspectrometer, wherein the impurities include SO₂; (d) directing a secondportion of the gas sample to an oxidizing furnace; (e) convertingreduced sulfur present in the second portion to SO₂; (f) directing gasexiting the oxidizing furnace to the gas cell; (g) measuring a total SO₂in the gas cell; and (h) repeating steps (a) through (g).
 12. The methodof claim 11, wherein step (c) is conducted at a pressure higher thanatmospheric and/or wherein step (g) is conducted at atmosphericpressure.
 13. The method of claim 11, further comprising switchingconditions in the gas cell between an atmospheric pressure and apressure above atmospheric.
 14. The method of claim 11, wherein totalSO₂ measured in the second portion of the gas sample is compared withSO₂ measured by the spectrometer in the first portion of the gas sampleto determine a TRS amount.
 15. The method of claim 11, wherein dataobtained for impurities in the first portion of the gas sample arecompared with calibration data.
 16. The method of claim 11, wherein theimpurities are selected from the group consisting of SO₂, total sulfur,NH₃, CO, NO, NO₂, H₂O, hydrogen cyanide (HCN), CS₂, methanol,acetaldehyde, methane, total hydrocarbons, benzene and total aromatichydrocarbons and the beverage grade gas is CO₂ or N₂.
 17. The method ofclaim 11, wherein the gas sample is selected from multiple gas samples,obtained from different locations in a plant, by a multiplexer.
 18. Themethod of claim 14, further comprising controlling gas pressure and/orgas temperature in the gas sample cell and/or purging the sample gascell after step (c).
 19. The method of claim 11, further comprisingmeasuring oxygen gas present in the sample gas.
 20. The method of claim11, wherein a CO₂% in the first portion of the gas sample is determinedby the spectrometer and compared to calibration information.
 21. Themethod of claim 11, further comprising controlling at least one of steps(a) through (h) automatically.
 22. The method of claim 11, wherein themethod does not include detecting SO₂ by UV fluorescence.